Millions of British households could claim an average of £117 for energy they have paid for but did not use over the winter, figures suggest.
More than 11 million households, or 42%, are in credit to their energy companies, and nearly one in 10 could reclaim more than £200, uSwitch said.
Just 14% of consumers - around 3.7 million households - have emerged from winter in debt to their supplier, a survey for the price comparison site found.
Under Ofgem regulations, consumers are entitled to any credit on request, as long as they have provided up-to-date meter readings.
But while households could reclaim £1.3 billion in total, 38% of consumers plan to leave their money in their account to cover price hikes.
Almost a third of customers (30%) say they will struggle to pay higher energy bills, while more than one in five (22%) say increases of £100 or more will force them into debt.
Millions of households paying by direct debit overpaid for their energy this winter as a result of warmer temperatures and steps to cut bills, according to uSwitch.
Two-thirds of customers (66%) took some form of action to cut their bills, such as turning down the thermostat (31%), turning down individual radiators (24%) and setting the heating to come on for less time every day (22%).
The Tory election manifesto is expected to include a promise to cut around £100 from energy bills by capping prices for the seven out of 10 households on standard variable tariffs.
Claire Osborne, uSwitch energy spokeswoman, said: "Under Ofgem rules, providers must repay any credit on request, so now is the time to read your meter, update your account and reclaim what you're owed.
"Consumers who are building up credit to soften the blow of price hikes should instead consider reclaiming this money and switching to a cheaper deal.
"With an average credit refund of £117 and savings of over £350 by switching supplier, it could be a very effective way to control spiralling energy bills."
:: Opinium surveyed 2,010 households online between March 31 and April 4.